Isobutylene-based polymers have been blended with numerous compounds such as natural rubber in order to improve its various properties, such as elasticity, strength, air impermeability, etc. Natural rubber (NR) is known to crystallize upon extension and is known to have very high molecular weight fractions, both of which help improve its green properties. The terms “green properties” and “green strength” are terms applied to denote the strength, cohesiveness and dimensional stability of rubber compounds before they are vulcanized or cured. Such properties are important in fabricating rubber articles from green compounds, particularly composites such as tires, but also can be important in extruded items such as innertubes and molded articles such as pharmaceutical stoppers. Isobutylene-based polymers are therefore blended with natural rubber when green properties need to be improved. However, green strength properties of isobutylene-based polymers are often adverse to those of natural rubber, particularly at elevated temperatures of up to 40 to 70° C. Addition of natural rubber reduces the barrier properties of isobutylene-based polymer/natural rubber blends significantly, which is undesirable for applications requiring low permeability to gases, such as in tires and in bladder applications. Heat stability of cured compounds is also diminished in natural rubber blends.
Isobutylene-based polymers, particularly halogenated isobutylene-based polymers, and more particularly brominated butyl rubber are the primary blends of most tire liners, heat resistant tubes, bladders and other commercially known products such as pharmaceutical ware. The term “butyl rubber” as employed herein is intended to refer to a vulcanizable rubbery copolymer containing, from 85 wt % to 99.5 wt % combined isoolefin derived units having from 4 to 8 carbon atoms. Such copolymers and their preparation are well known. See, e.g., RUBBER TECHNOLOGY 284-321 (Chapman & Hall 1995). Halogenated butyl rubber, particularly brominated butyl rubber, is also well known. It may be prepared by treating a solution of butyl rubber, in an organic solvent, with bromine and recovering the brominated butyl rubber by contacting it with steam and drying the resulting aqueous slurry.
Brominated butyl rubber typically contains less than one bromine atom per carbon-carbon double bond originally present in the polymer or from less than 3 wt % (weight percent) bromine. The Mooney viscosity of the halobutyl rubbers useful in the instant invention, measured at 125° C. (ML 1+8), range from 20 to 80, more preferably from 25 to 55, and most preferably from 30 to 50. It is a relatively chemically resistant, rubbery polymer which can be compounded and cured to produce synthetic rubber with an outstanding air impermeability, useful in making tire innerliners and innertubes.
Brominated butyl rubber has a greater degree of reactivity than butyl rubber, so that it can be blended with other unsaturated polymers and co-vulcanized therewith, which the unreactivity of butyl precludes. Brominated butyl rubber vulcanizates, however, show good air impermeability, heat aging characteristics and general chemical resistance. It finds one of its principal uses in the tubeless tire innerliners. Such liners are in effect thin sheets of rubber, adhered to the tire carcass by co-vulcanization with the rubbers comprising the tire carcass. The heat aging characteristics air impermeability and co-vulcanizability of brominated butyl rubber render it suitable for use in such tire innerliners. Other known uses for halogenated butyl rubber include white sidewall compounds for tires, heat resistant tubes and bladders.
A deficiency of butyl and halobutyl rubber is its lack of green strength when alone. In addition, the elongation characteristics of the uncured compounds can be used as a valuation of green strength. Lack of green strength renders difficult the processing and molding of rubber compounds based on butyl rubber. Green strength, viscosity and elastic memory are important properties influencing the processability of polymers and compounds in various end-use applications, e.g., tire fabrication. For example, in the manufacture of tire liners, very thin sheets of butyl rubber compound have to be prepared, applied to the green tire carcass and then cured. If the butyl or halobutyl rubber compound is deficient in green strength, there is risk of rupturing the thin sheets during processing unless very careful handling thereof is undertaken.
U.S. Pat. No. 4,256,857 discloses the improvement of green strength by treating the brominated butyl rubber with relatively small amounts of certain organic amine compounds. Examples of suitable amine compounds include N,N-dimethyl hexylamine, N,N-dimethyldodecylamine, N,N-dimethyloctadecylamine, N,N-diethyldecylamine and N,N-dimethylbenzylamine. These amine compounds have been found to provide green strength and allow the retention of good processing properties. While other amine compounds may be reacted with brominated butyl rubber to improve the strength of the rubber compound they generally also cause the rubber compound to be of inferior processing properties. In either case, heating and time requirements that are not efficient or practical for quick application for compounding in industrial applications.
U.S. Pat. No. 5,162,409 to Morocskowski describes a rubber blend suitable for use in automobile tire treads wherein the blend comprises a halogenated isobutylene rubber which can be the sole rubber of the blend or one of a combination of rubbers. A preferred embodiment comprises a rubber component comprising 20 to 60 wt % styrene/butadiene rubber, 20 to 60 wt % butadiene rubber, and 10 to 30 wt % of a halogenated rubber, a silica filler, and an organosilane cross-linking agent. It is disclosed that in a preferred embodiment, the rubber blends comprise 10 to 30 parts per 100 parts rubber (phr) of untreated, precipitated silica employed with an effective amount of organosilane coupling agent, for example, 1 to 8 phr. However, the green strength properties of the isobutylene rubber or blends thereof are not significantly improved.
The prior art has not addressed the full complement of green strength properties. In particular, what is needed is a blend that has improved relaxation and other properties to allow the blend to be processed at elevated temperatures, for example, at around 50° C., or from about 40 to 70° C. The present invention provides for a novel blend which addresses the present need for improved green strength while maintaining adequate impermeability.